This collaborative research program combines state-of-the-art mild chemical methods for peptide and small protein synthesis with biochemical and biophysical studies in order to address fundamental questions and develop hypotheses about protein folding, stability, and dynamics. The parent small globular protein targets are bovine pancreatic trypsin inhibitor (BPTI), 58 residues with an array of three disulfide bridges, and the immunoglobulin binding domain of streptococcal protein G (GB1), 56 residues without disulfides. During the first period of support, it was shown that replacement of cystine crosslinks in BPTI by paired alpha-amino-n-butyric acid (Abu) isosteres provides proteins which, under mild conditions, assume structures very similar to the ensemble of transient intermediates formed during the first 10-20 msec of the protein folding process. The earlier findings provide the underpinnings for the central hypothesis of the present proposal: in globular proteins, core motifs can be identified, and their elements can be combined in suitable peptides to construct native-like modules. The designed peptides, approximately 15-50 amino acid residues in length, consist of core elements (from BPTI and/or GB1) linked by natural or designed sequences, and they contain a strategically placed crosslink to limit conformational space to more collapsed conformations. The crosslink is designed with the ideal that its primary function is to restrict the mobility (entropy) of the chain, rather than to stabilize folded structure. Core modules are of great interest in themselves, and it is proposed to characterize their conformational ensembles and to optimize their stabilities of their native-like conformation(s). Further, strategies have been developed to link core modules in tandem to construct a larger protein with a true native state. Integrated throughout this research are continued improvements and optimizations of chemistry and purification tools for efficient preparation of homogeneous small protein analogues, including issues involved with the efficient creation of disulfide, thioether, side-chain lactam, and head-to-tail lactam crossbridges. The ongoing and proposed studies exemplify new approaches and are leading to significant and generalizable insights that contribute to the "protein folding problem."